1. Field of the Invention
The invention relates to an electrophotographic photosensitive member adapted for use in a printer, a facsimile apparatus, or similar electrophotographic type device utilizing a laser beam as an exposure light source, and more particularly to an electrophotographic photosensitive member improved with respect to generation of interference fringes and other electrophotographic characteristics.
2. Background Art
In the invention, a printer or facsimile apparatus of the electrophotographic type preferably utilizing a laser beam as an exposure light is typified by a dry-process electrophotographic apparatus utilizing the electrophotographic process of C. F. Carlson, in which an internal electrophotographic photosensitive member is charged on a surface thereof and is subjected to exposure to form an electrostatic latent image, toner supplied from a developing device is electrostatically deposited thereon under the application of a bias voltage in a developing step, and the toner is then transferred onto paper in a transfer step, thereby obtaining an image.
In such a dry-process electrophotographic apparatus, optical interference is often generated because of the use of a coherent (monochromatic) laser beam as the exposure light source. An interference fringe pattern, such as a moiré pattern or a zebra pattern formed on a printed output image, deteriorates the image quality.
The interference fringe pattern is generated because monochromatic light reflected at a top surface of a photosensitive layer interferes with light reflected at interfaces of internal layers, including a surface of a substrate, causing optical interference due to unevenness in the layer thickness, which leads to an undulating intensity of the reflected light. In an electrophotographic photosensitive member, the largest influence is usually interference between the laser beam reflected at the surface of the conductive substrate and the laser beam reflected at the outermost surface of the photosensitive layer.
Various proposals have been made regarding the drawback of such interference fringes. It is already known, for example, to form fine irregularities on the surface of the conductive substrate by a sand blasting process, in order to cause random reflection and reduce the amount of light reflected in a particular direction, thereby suppressing or preventing the interference fringes. This is described in Japanese patent publications JP-A-2001-75299, JP-A-2001-249477, JP-A-2000-66428 and JP-A-2000-75528.
Japanese patent publications JP-A-2002-296822 and JP-A-2002-296824 describe the use of a conductive substrate having a roughened surface, which is obtained by sampling surface roughness data for a predetermined area, determining a power spectrum by a Fourier transformation, and roughening the surface so as to obtain plural peaks, and to form a photosensitive layer thereon. The resultant photosensitive member is free from image streaks.
Japanese patent publication JP-A-2002-174921 describes a photosensitive member utilizing a conductive substrate on which surface roughness is regulated to a predetermined average surface roughness and a predetermined maximum surface roughness.
The aforementioned methods for preventing interference fringes by forming fine irregularities on the surface of the conductive substrate are associated with the following drawbacks. The present inventors have conducted investigations with an organic photosensitive member in which an undercoat layer and a photosensitive layer formed by coating on a sand blasted conductive substrate have a layered structure including an undercoat layer, a charge generation layer, and a charge transport layer (such a photosensitive member being hereinafter called a “laminate organic photosensitive member”). They found that with such an organic photosensitive member the methods described in the aforementioned patent references have a certain effect for preventing interference fringes. However, they also found that the fringes cannot be eliminated to a satisfactory level by simply working the conductive substrate to a predetermined surface roughness.
For example, even on a conductive substrate which is appropriately sand blasted to prevent the interference fringes, an increase in the thickness of the undercoat layer formed by coating again gradually onto such a conductive substrate enhances interference fringes on the image. It is also known that, for such a thicker undercoat layer, the interference fringes can be suppressed to a certain extent by employing a conductive substrate in which the sand blasting treatment is applied stronger to obtain larger surface irregularities corresponding to the thickness of the undercoat layer. However, for confirming the presence or absence of interference fringes, it is necessary to: coat a photosensitive layer on the undercoat layer thereby obtaining a photosensitive drum; then to mount necessary components such as flanges on both ends of the drum; mount the photosensitive member on an image forming apparatus (actual apparatus); and to form an image. Such a method has the drawbacks of requiring time and labor and being unable to provide a result immediately. Furthermore, there is another drawback in that the interference fringes cannot always be prevented by the roughening the surface of just the conductive substrate, because s conditions inducing the interference fringes are also affected by the photosensitive layer formed on the undercoat layer.
Based on the foregoing, an analysis of the factors causing the interference fringes has led to the following consideration. In the case where an organic photosensitive member, formed by laminating a charge generation layer and a charge transport layer in succession as a photosensitive layer onto a conductive substrate provided with an undercoat layer, is irradiated with a monochromatic laser beam, the charge generation layer usually employs organic pigment particles capable of absorbing a coherent wavelength of the laser beam, as the required charge generation material. Such organic pigment particles, usually being insoluble in organic solvents, are uniformly dispersed in a resinous binder to form the charge generation layer of a satisfactory charge generating function.
A laser beam entering the photosensitive layer of such photosensitive member, upon passing the upper charge transport layer and reaching the charge generation layer, is partially absorbed by the charge generating material but also passes through the gaps between the particles of the organic pigment, thus irradiating the undercoat layer. The light irradiating the undercoat layer is divided into a light intruding into the undercoat layer and a light reflected from the surface of the undercoat layer. The light reflected at the surface of the undercoat layer reaches the outermost surface of the photosensitive layer and in turn is divided into a component outgoing from the outermost surface and a component reflected inwards at the outermost surface and directed again to the charge generation layer.
On the other hand, the light intruding into the undercoat layer, upon reflection by the conductive substrate, enters the charge generation layer again and is divided into a component absorbed by the organic pigment in the charge generation layer and a component passing through the gaps between the organic pigment particles to reach the outermost surface of the photosensitive layer. Some of this latter component goes out from this outer surface, and some is reflected inwards at the outermost surface and directed again to the charge generation layer.
Thus, in the photosensitive layer, an entering laser beam not only intrudes into a certain layer but also has a component reflected at a surface of such layer. Such reflected a component significantly influences the optical interference. A detailed consideration clarifies that the reflections which contribute to interference fringes include reflections from plural surfaces.
A first reflection factor is a superposition of a first component of the laser beam, reflected at the surface of the undercoat layer, then reaching the outermost surface of the photosensitive layer and going out to the exterior and a second component of the laser beam, reaching and reflected at the outermost surface of the photosensitive layer. The optical intensity becomes stronger in such superposing position and weaker in a non-superposing position thereby generating an unevenness in the optical intensity and causing interference fringes on the image.
Another reflection factor is interference between a light reflected at the surface of the conductive substrate and a component of the entering light reflected at the outermost surface of the photosensitive layer.
Thus, the interference fringes are principally generated in two modes, namely by a reflection from the surface of the conductive substrate and by a reflection from the surface of the undercoat layer.
The foregoing explanation on the generation of interference fringes indicates that (in the case of an organic photosensitive member having a photosensitive layer, across an undercoat layer, on a conductive substrate with a roughened surface formed by a sand blasting process) a complete prevention of interference fringes is difficult to achieve through a simple reduction of the reflection intensity in a particular direction from the surface of the conductive substrate by a random reflection caused by surface roughening, unless the surface of the undercoat layer is also roughened. In that case, the influence of the light reflected from the surface of such undercoat layer is not negligible (for example, in case a thick undercoat layer is formed).
However, in the case of most prior members, the roughening of the surface of the undercoat layer need not be considered. This is because when a thin undercoat layer is formed by coating, the surface of such an undercoat layer spontaneously has irregularities following the irregularities of the sand blasted surface of the conductive substrate, even without additional roughening of the surface of such a thin undercoat layer.
Nevertheless, a thin undercoat layer decreases the total film thickness after the formation of the photosensitive layer, thus reducing electrical resistance across the total film thickness. This structure results in a drawback of easily causing a leak in the photosensitive layer during the charging process, particularly in an image forming apparatus, such as a printer, utilizing a contact charging process. Since such a leak causes a trace of the leak on the image or a stripe-shaped image defect having a periodicity of the drum periphery, a thick undercoat layer has been required for an electrophotographic photosensitive member for use in a printer or the like.
In the case of forming a thick undercoat layer by coating, in order to prevent the aforementioned leak phenomenon, it is necessary, as explained before, to employ a conductive substrate of a surface roughness with enlarged irregularities (a larger average roughness Ra and a larger maximum surface roughness Rmax) so that the roughened state of the substrate surface is reproduced on the surface of even a thick undercoat layer, or to later roughen the surface of the undercoat layer. However, the former method is limited because the layer cannot be made very thick. Also, the latter method of also roughening the surface of the undercoat layer leads to new drawbacks, namely, that fogging or a leak in the form of black spots on a white background tends to be generated corresponding to protruding parts on the surface of the undercoat layer, and that uneven density corresponding to the irregularities results in a halftone image.
Still another cause for interference fringes is deviation in the film thickness of each of the undercoat layer, the charge generation layer, and the charge transport layer. Among these, deviation in the film thickness of the charge transport layer, constituting the outermost surface of the photosensitive layer, has the largest influence. This is because the charge transport layer usually has the largest thickness, thus constituting the largest factor generating such deviation in the film thickness. As to the thickness deviation of the charge transport layer, for a semiconductor laser of a wavelength of 780 nm, a theoretically zero deviation is not necessary required, and the interference fringes of a practically unacceptable level are not generated at a film thickness deviation of 0.3 μm or less.
An experiment was conducted for confirming a correlation between the film thickness deviation and the generation of interference fringes. The experiment utilized a photosensitive drum, having a coated charge transport layer with a film thickness deviation of 1-5 μm and formed by employing a non-sand blasted conductive mirror-surface substrate (plain pipe), a coating liquid having a viscosity for forming a charge transport layer, and a seal coating method which tends to generate film thickness deviation, in order to intentionally cause interference fringes. In this experiment different interference fringe patterns were obtained according to the film thickness deviations. In order to prevent interference fringes with such mirror-surfaced plain pipe, it is at least necessary to employ a dip coating method capable of providing little film thickness deviation, thereby maintaining the film thickness deviation of the coated charge transport layer within a printing area in the axial direction and the circumferential direction of the drum at 0.3 μm or less. In practice, however, the dip coating usually provides a film thickness deviation of 0.5 to 3 μm/axial direction, or 0.5 to 1.5 μm/axial direction even under a careful operation, so that a film thickness deviation of 0.3 μm or less is, even if possible experimentally, difficult to achieve in an effective mass production. Therefore, interference fringes have been prevented by employing, as a conductive substrate as described in the aforementioned patent references, a substrate (plain pipe) which is roughened to a predetermined roughness by a sand blasting instead of a mirror-surface substrate.
In order to achieve a stable mass production of an electrophotographic photosensitive member free from interference fringes, there is desired a production method capable of avoiding interference fringes even with a somewhat larger film thickness deviation, rather than aiming at a reduction in the film thickness deviation<which is extremely difficult to achieve in mass production.
It is known, as described in the foregoing Japanese patent publications JP-A-2002-296822 and JP-A-2002-296824, to determine the relation of a substrate with a roughened surface to generation of interference fringes by sampling surface roughness data of a predetermined area and by obtaining a power spectrum through Fourier transformation, but such a relation is a function only of the surface roughness of the substrate. As will be explained later, generation of the interference fringes is also affected by conditions for forming the undercoat layer and the photosensitive layer on the substrate.